Normally, intervertebral discs, which are located between endplates of adjacent vertebrae, stabilize the spine and distribute forces between the vertebrae and cushion vertebral bodies. The discs may be displaced or damaged due to trauma, disease or aging. A herniated or ruptured annulus fibrosis may result in nerve damage, pain, numbness, muscle weakness, and even paralysis. Furthermore, as a result of the normal aging processes, discs dehydrate and harden, thereby reducing the disc space height and producing instability of the spine and decreased mobility. Most typically surgical correction of a disc space includes a discectomy (surgical removal of a portion or the entire intervertebral disc.) The discectomy is often followed by preparation of the vertebral endplates to receive an implant, such as a fusion device. Fusion of the adjacent vertebrae is used to alleviate the pain associated with abnormal joint mechanics, premature development of arthritis, and damage to the spinal structures.
Traditional surgical procedures for correction of spinal defects can cause significant trauma to the nerves and vasculature adjacent the spine. The potential for tissue trauma can be of particular concern when the surgical procedure dictates the use of a cutting instrument in the disc space in order to remove disc and bone material to prepare the disc space for a fusion device, intervertebral spacer, or other implant. In order to minimize the potential trauma, surgical procedures can require denervation and devascularization of tissue adjacent the disc space. Other procedures require manipulation of the nerves and vasculature in order to provide clear access to the disc space. While reducing the potential for contact with surgical instruments, manipulation and removal of the nerves and vasculature can still result in trauma and an increase the time and difficulty of the surgical procedure.
U.S. Pat. No. 5,722,977 to Wilhelmy, incorporated herein by reference in its entirety, discloses a method and instrumentation for removal of a pair of rectangular blocks of vertebral bone tissue to provide an intervertebral recess. The instrumentation includes an intervertebral mortising cutter and a spacer that acts as a guide for the cutter. The procedure first requires the spacer to be inserted between the vertebral bodies. The cutter has a cutting head that defines a cavity that receives the spacer. In order to remove the bony material, the cutting head is pushed around the spacer and into the disc space. One disadvantage with the instrumentation disclosed in the '977 patent is that the cutting head is positioned between the spacer and the adjacent nerves and vasculature.
U.S. Pat. No. 5,484,437 to Michelson discloses a technique and associated instrumentation for inserting a fusion device from a posterior surgical approach that provides protection for the surrounding tissues and neurological structures during the procedure. As described in more detail in the '437 patent, the surgical technique involves the use of a distractor having a penetrating portion that urges the vertebral bodies apart to facilitate the introduction of the necessary surgical instrumentation. The '437 patent also discloses a hollow sleeve having teeth at one end that are driven into the vertebrae adjacent the disc space created by the distractor. These teeth engage the vertebra to maintain the disc space height during subsequent steps of the procedure following removal of the distractor. In accordance with one aspect of the '437 patent, a drill is passed through the hollow sleeve to remove portions of the disc material and vertebral bone to produce a prepared bore for insertion of the fusion device. One disadvantage with this technique is that the procedure is performed through a sleeve, thus limiting working space and visibility of the surgeon. A further disadvantage is that the height of the prepared disc space is limited by the spacing between penetrating portions of the distractor and the inner diameter of the sleeve.
While the more recent techniques and instrumentation represent an advance over earlier surgical procedures for the preparation of the disc space for insertion of implants between the vertebrae, the need for improvement still remains. The present invention is directed to this need for improvement and provides convenient methods and instruments for safe and effective spinal surgical procedures.